The following paragraphs contain some discussion, which is illuminated by the innovations disclosed in this application, and any discussion of actual or proposed or possible approaches in this Background section does not imply that those approaches are prior art.
Natural resources such as oil and gas residing in a subterranean formation or zone are usually recovered by forming a wellbore that extends into the formation. The wellbore is drilled while circulating a drilling fluid therein. The drilling fluid is usually circulated downwardly through the interior of a drill pipe and upwardly through the annulus, which is located between the exterior of the pipe and the walls of the wellbore. After terminating the circulation of the drilling fluid, a string of pipe, e.g., casing, is run in the wellbore. Next, primary cementing is typically performed by pumping cement slurry into the annulus and allowing the cement to set into a hard mass (i.e., sheath). The cement sheath attaches the string of pipe to the walls of the wellbore and seals the annulus.
Often in drilling a wellbore, one or more pervious zones are encountered. The pervious zones may be, for example, highly permeable, unconsolidated, vugs, voids, naturally occurring fractures, or induced fractures that occur when weak zones have fracture gradients exceeded by the hydrostatic pressure of the drilling fluid or the cement slurry. During the drilling operation, the pervious or thief zones may result in the loss of drilling fluid. The drilling fluid flows into the thief zones rather than being returned to the surface, which reduces circulation of the drilling fluid. When circulation is lost, pressure on the open formation is reduced, which can result in an undesired zone flowing into the well or even catastrophic loss of well control.
A large variety of materials have been used or proposed in attempts to cure lost circulation. Generally, such materials are divided into four types or categories: fibrous materials, such as monofilament synthetic fibers; flaky materials, such as wood chips or mica flakes; granular materials, such as ground marble or petroleum coke; and settable compositions, the relative strength of which increases upon a preplanned mode of triggering after placement, such as hydraulic cement.
Although many materials and compositions exist and have been proposed for preventing lost circulation, there continues to be a need for even more versatile and better compositions and methods for preventing, as well as mitigating, loss of circulation.